This invention is directed to middle distillate production (e.g., diesel and kerosene products) by means of a reactor hydroprocessing system using two or more reactors (or a single reactor vessel having two or more stages, each stage containing one or more reaction zones). Product effluents are effectively segregated to avoid recracking of products, to dramatically reduce hydrogen consumption in saturating the bottoms product, and to carry out aromatic saturation of middle distillates in a clean low-temperature environment.
In an SSOT (single-stage once-through) environment, all the products of the reaction from each zone of a reactor are forced to pass over following zones in a cascade mode. Operating conditions of the reactor are dictated by the need for deep denitrification and subsequent conversion in a harsh ammonia and hydrogen sulfide-rich environment. Temperatures tend to be higher, favoring hydrocracking, and are not optimal for aromatic saturation. Recracking occurs in the lower beds, leading to destruction of valuable diesel and jet range material to naphtha and lighter material. Since there is no subsequent reactor stage available, all products must be hydrogenated in the same reactor system. The biggest source of hydrogen loss is the oversaturation of the unconverted oil destined for the FCC unit.
The parent application was concerned with a single stage process (employing more than one reaction zone, preferably in a single reactor vessel) for hydroconverting dissimilar refinery streams using a single hydrogen source. It disclosed a method for hydroprocessing two refinery streams using a single hydrogen supply and a single hydrogen recovery system. It further disclosed a method for hydrocracking a refinery stream and hydrotreating a second refinery stream in a common reactor and with a common hydrogen feed supply in which the feed to the hydrocracking zone was not poisoned with contaminants present in the feed to the hydrotreating reaction zone. Furthermore, the parent application was directed to hydroprocessing two or more dissimilar refinery streams in an integrated hydroconversion process while maintaining good catalyst life and high yields of the desired products, particularly distillate range refinery products. Such dissimilar refinery streams might originate from different refinery processes, such as a VGO, derived from the effluent of a VGO hydrotreater, which contains relatively few catalyst contaminants and/or aromatics, and an FCC cycle oil or straight run diesel, which contains substantial amounts of aromatic compounds.
Publications concerned with methods for using a single hydrogen loop in a two-stage reaction process have been disclosed in the parent application. The instant invention is further concerned with effectively segregating reaction stages in order to avoid recracking of products. Segregation may be done using two separate fractionation columns or a single fractionation column in which reaction stages are separated by the use of a baffle. The article, xe2x80x9cDivided-wall columns novel distillation conceptxe2x80x9d (Process Technology, Autumn, 2000), discloses the use of divided wall columns in benzene removal processes.
WO 97/23584 discloses an integrated hydroprocessing scheme involving a hydrocracking stage and a subsequent dewaxing stage for the production of lubricants, as well as naphtha and middle distillates. (The instant invention is directed to hydrocracking and hydrotreating of middle distillates). The bottoms streams, and optionally other streams from each stage, are maintained separately from one another during processing. Dewaxing may occur using either hydroisomerization catalysts, shape-selective catalysts, or both in series. One embodiment employs a baffle in the flash zone of a fractionator to separate bottoms streams from each other. Alternately, the effluent from the hydrocracking stage may be processed separately from the effluent from the dewaxing stage. The bottoms fraction from the dewaxing stage may be recycled back to the hydrocracking stage for further processing or used as a lube base stock.
This invention is directed to middle distillate production (e.g., diesel and kerosene products) by means of a reactor hydroprocessing system using two or more reactors (or a single reactor vessel having two or more stages, each stage containing one or more reaction zones). Hydrocracking is preferably performed in the initial reactor, and hydrotreating (and/or further hydrocracking) is preferably performed in a subsequent reactor or reactors. Reaction effluents are effectively segregated to avoid recracking of products, in order to dramatically reduce hydrogen consumption in saturating the bottoms product and to carry out aromatic saturation of middle distillates in a clean low-temperature environment.
The quality of the products from the different reactors (or stages) can be distinctly different, and this invention keeps them segregated for specialized use or marketing. The preferred means of separation is by using separate fractionators or distillation columns, although, in an alternate configuration, a single fractionator having a baffle may be used. The latter configuration results in decreased modification expense.
In the instant invention, when hydrotreating is desired, feed may be hydrotreated at relatively high space velocities and low hydrogen-to-oil ratio. Conditions will be suitable for deep hydrodesulfurization, hydrodenitrification and low conversion. Intermediate flash zones and rough fractionation segregates the lighter product effluent from the first reactor from the bottoms.
FCC feed essentially consists of unconverted oil from the first reactor. The remainder of the unconverted oil is extinction cracked to diesel in a clean second stage reactor operating under typical second stage hydrocracking conditions. The last bed of the second stage reactor is used to xe2x80x9cpost-treatxe2x80x9d the small quantity of distillates formed in the first stage.
The operating conditions in the second reactor (or stage) of a two-reactor (or two-stage) hydroprocessing system (moderate temperature, high partial pressure hydrogen, low partial pressure nitrogen, and low partial pressure H2S) are very favorable for aromatic saturation. Therefore, injection of middle distillates or other stocks needing saturation into the bottom beds and processing over treating catalyst (the second-stage cracking catalyst being upstream or mostly upstream of the point of injection) provides a low cost means to upgrade these stocks. The injected stocks might be straight run kerosene or diesel, cracked stocks such as coker gas oils or FCC cycle oils, or could even be first stage middle distillates in cases where first stage conditions hinder the attainment of what are sometimes very stringent product specifications (e.g., smoke point, cetane number). This scheme can also be used for very deep hydrodesulfurization.